Theoretical Studies of Catalysis by Carboxypeptidase A: Could Gas-Phase Calculations Support a Mechanism?

We compare recent quantum mechanical computations of alternative reaction pathways for carboxypeptidase A, a zinc proteinase, in an "enzyme environment" to similar calculations in the "gas phase" that include the minimal chemical entities that are required for a non-catalytic reaction. The main question that we address is whether anything may be learned from such reduced representations. Two general acid-general base alternative pathways and one nucleophilic pathway are compared. The original calculations were run on a relatively large model (120 atoms) of the active site of carboxypeptidase A which included zinc and its ligands, as well as the residues Arg145, Arg127, Glu270, a water molecule and a model dipeptide. The "gas-phase" pathways include only the dipeptide, water and Glu270 and serve as models for the non-catalytic pathway. The calculations were performed by semiempirical MNDO/H/d that includes modifications for d-orbital representations as well as for intra- and intermolecular multiple H-bond formation. The gas-phase results strengthen our previous conclusion about the preference for general acid-general base pathways for peptide cleavage by carboxypeptidase A rather than a "direct nucleophilic" pathway. The bottleneck of the reaction is proton transfer to the nitrogen in the peptide bond, preceding the peptide cleavage.